The present invention relates to electronic semiconductor devices and methods of fabrication, and, more specifically, to gallium arsenide heterojunction bipolar devices that have planar structure suitable for large scale integration.
Semiconductor devices made of gallium arsenide are preferred over devices made of silicon for high frequency applications due to the higher mobility of carriers in gallium arsenide; however, gallium arsenide material and fabrication technology lag far behind that of silicon. Indeed, gallium arsenide MESFET integrated circuits with more than a 1,000 gates have been fabricated (for example, Toyoda et al, A 42ps 2K-Gate GaAs Gate Array, 1985 ISSCC Dig. Tech. Papers 206), but the precise control of device parameters such as threshold voltage for larger scale integration has not yet been achieved. Similarly, HEMTs, which use the two dimensional electron gas at a heterojunctions of gallium arsenide and aluminum gallium arsenide, provide fast devices but suffer from the lack of precise control of device parameters; see Mimura et al, High Electron Mobility Transistors for LSI Circuits, 1983 IEDM Tech Digest 99.
Bipolar transistors have several advantages over FETs for high speed applications, and, for example, the turn-on voltage is determined by physical parameters and is not sensitive to the geometry and doping levels as is the threshold voltage o MESFETs or HEMTs. However, the fabrication of gallium arsenide bipolar (or heterojunction bipolar) transistors suitable for high speed applications, such as ECL, is complicated by the need for contacts to buried collector layers and isolation by mesa structures; see, Asbeck et al, 1984 GaAs IC Symp. Tech. Digest 132. Such non-planar devices are incompatible with high level integration. Consequently, attempts have been made to overcome these problems (Taira et al, New Lateral GaAs Transistor, 1984 IEDM Tech Digest 201), but the solutions are not satisfactory.